Visible lasers are becoming increasingly important for many application areas. In healthcare and medical research, visible lasers are used for oncologic photodynamic therapy, glucose level monitoring, prostate ablation, gene mapping and chromosome sorting, and other medical treatments and biomedical diagnostics. In environmental monitoring and protection, visible lasers are used for bathymetric (oceanic) sensing and monitoring, large-scale algae mapping, geological activity monitoring, undersea optical data transmission, and artificial guide stars for trans-atmospheric imaging. High-tech commercial applications may need visible lasers for three-dimensional displays, high-density optical storage, or high-speed, high-resolution printing.
Many of these applications require high powers, ranging from 100 mW for medical diagnostics and data storage to 10's of Watts for artificial guide stars and laser displays. Unfortunately, gaining access to reliable visible laser wavelengths at high power has proven difficult, even though many materials appear suitable for producing visible lasers. Common paths to visible lasers include gas and liquid lasers, semiconductor and exotic glass lasers, and nonlinear conversion methods.
Gas lasers, dye lasers, and optical parametric oscillators (OPOs) are cumbersome, inefficient, and require continual alignment, making them poor candidates for any application outside the laboratory environment. Second harmonic generation (SHG) and other specialty crystals can only reach limited visible wavelength bands, while upconversion techniques are inefficient.
Semiconductors are a laser material system that can cover the entire visible band. Recent advances in GaN semiconductor lasers have allowed direct access to visible wavelengths, ranging from red to blue and into the UV. Like most semiconductor lasers, however, obtaining powers substantially beyond 10 mW becomes increasing difficult due to thermal stress, facet damage, and beam break-up due to filamentation, making them unsuitable for many high-power (>100 mW) applications.
Most high-power visible lasers are frequency-doubled solid-state or semiconductor lasers. The prime example seen in green laser pointers is Nd:YAG doubled to 532 nm. Frequency doubled lasers are inefficient, as well as limited in wavelength. Upconversion lasers are similarly inefficient. Fiber lasers based on non-silica glasses are typically difficult to fabricate and fragile to handle.
Fiber and other solid-state lasers offer practical benefits in terms of reliability and efficiency. Fiber lasers in particular offer compact packaging, extremely robust all-fiber (alignment free) cavities, and heat dissipation well beyond the 100 W level. Moreover, advances in telecommunications and high-power infrared (IR) fiber lasers have pushed the silica fiber platform to unprecedented heights of technological infrastructure, including pump packaging, all-fiber components, and custom fiber fabrication with specifications nearly identical to high-end commercial fabrication. These advantages make fiber lasers the ideal general laser platform.
Recent work in direct (i.e., not upconversion) visible fiber lasers has utilized rare earth dopants of Pr, Dy, Sm, and Tb. As will be described later, most of these efforts required the use of fibers in exotic materials such as fluoride-based ZBLAN, which is brittle, difficult to fabricate, difficult to handle and process, and in general not suitable for the common highly developed fiber optical platform (silica). In other words, such a specialty fiber, by not being compatible with the silica platform, no longer carries the significant advantages commonly touted for “fiber lasers.”
Rare earth (RE) dopants are highly soluble in glass and therefore suitable for fiber lasers. Most RE ions have one or more atomic transitions that produce visible emission. However, there are many issues prohibiting the use of many of these elements. The most prominent in determining which material system and RE ion to use is the issue of multiphonon emission. Although a given energy level may in general have a large lifetime suitable for high efficiency lasing characteristics, this lifetime can be significantly reduced depending on the host (glass matrix) that it is doped into. The excited electron can interact with the multiple phonons of the host medium to extract sufficient energy to reduce the electron down to the next available energy level. Since the transition probability for such an interaction decreases exponentially with the number of phonons required for the interaction, it is desirable to use a RE ion with an upper (meta-stable) fluorescing level that is high above the next lower energy level. From this perspective, it would seem that terbium is a good candidate for a lasing RE ion, since the 5D4 level is 14 cm−1 above the next lower level. This energy level spacing is larger than all other RE ions with visible optical transitions, and more than twice the energy separation than all other RE ions except europium.
The energy separation is not the only factor in determining the strength of multi-phonon interactions. The material composition of the host medium that supplies the phonons is critical. Since the meta-stable fluorescence rate is on the order of a few milliseconds for most RE ions, having a host medium with a similar or shorter multi-phonon emission rate serves to reduce the upper-state lifetime drastically. For the same energy gap, ZBLAN (fluoride fiber) has a multi-phonon emission rate that is 1,000× lower than that of silica fiber. For this reason, ZBLAN has been a material chosen for visible fiber lasers despite the inability for ZBLAN to mesh with the highly developed silica infrastructure.
However, the benefits of lower multi-phonon emission of ZBLAN may be regained by using terbium as the RE ion. Experiments in visible emission used praseodymium, dysprosium, and samarium as the RE ions. The energy gap from the nearest lower energy level is on the order of 7,000-8,000 cm−1 for each of these ions. In terbium, the energy gap is twice as large (˜15,000 cm−1). Given the extreme slopes of the multi-phonon emission curves, the multi-phonon emission rate can be lowered by well over a factor of 1,000 by using terbium, potentially allowing the silica platform to be exploited.
Terbium RE ions have additional benefits over other RE ions with visible transitions. First, Tb3+ has emission lines that span a significant range of visible light with regards to visual color discrimination. FIG. 1 shows the photoluminescence emission spectra of highly Tb3+ doped (56-wt %) fiber excited with UV (405 nm) light. FIG. 1 demonstrates that nearly the entire color perceptive scale (except violet) can be achieved using a single gain medium. This has tremendous implications for high-power, energy-conserving laser displays.
FIG. 1 also clearly demonstrates the potential for low-cost, high-power Tb:fiber lasers for use in bathymetry (480-550 nm), artificial guide stars (589 nm), oncologic photodynamic therapy (620-630 nm), and the other applications previously listed. Moreover, there are more accessible visible emission bands in Tb than in Pr or Dy. This makes Tb highly attractive as a platform for research and development as it can cover a much wider range of the visible spectrum than any other RE element.
An additional benefit from terbium comes from its absorption characteristics. FIG. 2 shows the absolute absorption spectrum of our highly Tb3+ doped (56-wt %) fiber. The measurement was performed using an arc lamp as a source, with the absorption spectrum calculated by dividing the output spectrum by the input spectrum. The resulting absorption, displayed in FIG. 2 in units of dB/m, has several distinguishing features. First, by using a multi-component silicate glass host, the absorption coefficient can be very large, well suited to the dual-clad fiber laser geometries that have led to kW-class fiber laser systems by allowing the use of inexpensive diode laser pumps. Second, there is a strong absorption peak at ˜488 nm, allowing for the possibility of pumping with frequency-doubled 976-nm pump lasers (commercially at the Watt level) or possibly upconversion cooperative energy transfer (CET) by co-doping the Tb:fiber with ytterbium that has a very strong 976-nm absorption peak. There is a strong and broad absorption peak centered around 400 nm. Violet diode lasers at 405 nm are currently undergoing significant development due to the Blu-ray DVD market, with commercially available individual lasers approaching the Watt level. As such, direct violet diode pumping of Tb:fiber lasers will ultimately enable high-efficiency generation of laser light at all perceptual visible wavelengths.
A factor that determines the quality of a specific laser material is the presence of excited-state absorption (ESA), wherein a pump photon gets absorbed, exciting an electron from the meta-stable state to an even higher energy state that cannot provide optical emission. This highly excited electron loses energy to multi-phonon or cascaded phonon interactions, sometimes even bypassing the meta-stable level entirely. In either case, the net result is loss for pump or signal (depending on which energy levels exhibit ESA properties), resulting in severely degraded laser efficiency. FIG. 3 shows a diagram of the relevant levels for visible lasing in terbium. Once an electron has been excited to the 5D4 level, the location of the 4f75d band allows this electron to be excited to yet a higher level. This additional imparted energy is lost to the host material via multi-phonon interaction, causing a net reduction in the meta-stable level population and an effective reduction in absorbed pump power.
Given the small-signal nature of the gain measurement, a linear trend is expected for gain as a function of increasing pump power. However, the practical implications of ESA result in the measured gain rapidly rolling over with increasing pump power, regardless of pumping architecture. The modeling indicates that this is due to ESA, as described above.
For this reason, ESA is a major impediment towards realizing visible fiber lasers using terbium. Avoiding ESA is one of the primary reasons that others have used praseodymium, dysprosium, and samarium instead of terbium to generate visible light from fibers. However, these materials nominally require ZBLAN fibers to minimize multi-phonon interactions, and as indicated above, working in a non-silica host is a major impediment in achieving robust and reliable fiber laser technology.
Therefore, what is needed is a new method of lasing with ESA-impaired laser materials and, more particularly, to mitigate ESA effects to allow the use of terbium, lambda-type materials, or other materials in a laser.